how to draw a line exactly in the middle of a polygon

[srinivasreddy]

I have a river polygon class and a empty polyline class i want to draw a polyline   between the two edges of the river polygon and add to polyline class.

Any body can give me the sugestions to do this,,,

Thank you

[Durga_PrasadD]

There are no out of box solutions to this problem.This is a research topic and there only optimal solutions. I came across this link [URL="http://www.spatialprojects.com.au/spatialsql_median.htm"]  and haven't got chance to experiment with it.

[Durga_PrasadD]

Thiessen polygons.

I tried to create a medial line with the procedure(Thiessen polygons) suggested .I do not seem to get  the medial line(attachment).Did I miss anything ?

[Durga_PrasadD]

Thank you whuber ,I got good result after using the densify.Nevertheless ,it invloved manual effort to remove additional lines.The effort involved is tremendous in case of higly concave polygons(attachment).